FIG. 1 illustrates a block diagram of a prior art system 10 for manually controlling the connection of an airframe mounted accessory drive (AMAD) 12 to a propulsion turbine 14 of a jet aircraft by means of an engine gearbox 13 and power takeoff (PTO) 15. This system is utilized in the F-18 jet aircraft to permit the selective connection and disconnection of the airframe mounted accessory drive 12 from the propulsive turbine engine 14 for purposes of testing the electrical generator 17 and hydraulic pump(s) 19 for producing on board power by a drive applied from an auxiliary power unit (APU) 21 with the engine gearbox 13 removed. The APU is conventionally used for starting of the propulsion engine 14 and may also drive the generator 17 and hydraulic pump(s) 19 in flight. The system 10 has a handle 23 which is attached to a coupling mechanism 25 which is axially movable between first and second positions at which the handle may be locked. In the first position, the airframe mounted accessory drive 12 is mechanically connected to the propulsion turbine 14 via engine gearbox 13 and PTO 15 and in the second position the airframe mounted accessory drive is disconnected from the propulsion turbine to permit drive by the APU 21 of the electrical generator 17 and hydraulic pump(s) 19. The handle is connected to a rack which engages a pinion gear which is connected to an acme screw within the coupling mechanism. Rotation of the acme screw controls the movement of a rotatable shaft within the coupling mechanism 25 between first and second positions at which the airframe mounted accessory drive 12 and propulsion turbine 14 are respectfully connected and disconnected from each other.
The prior art system has a number of disadvantages. In the first place, the coupling mechanism 25 including the acme screw and rack and pinion are expensive to manufacture. Furthermore, the rack and pinion drive coupled to the acme screw is characterized by substantial backlash which makes it difficult to monitor the precise position of the coupling mechanism 25 in the first and second positions which is essential for safe operation of the aircraft. It is of extreme importance that false readings are not possible in monitoring the position of the coupling mechanism 25 in the first and second positions for the reason that the inadvertent disconnection would prevent the airframe mounted accessory drive from being used to generate on board electrical and hydraulic power which is necessary for proper aircraft operation.